1. Field of the Invention
Embodiments of the present invention generally relate to semiconductor processing, and particularly to monitoring solid precursor delivery to a semiconductor process chamber.
2. Background of the Related Art
As integrated circuit (IC) density increases, the need for greater uniformity and process control regarding layer thickness rises. IC fabricators make aggressive demands on the semiconductor processing industry to develop fabrication tools that provide for larger production yields while increasing the uniformity of layers deposited on substrates having increasingly larger surface areas. In response to these demands, various technologies have been developed to deposit layers on substrates in a cost-effective manner, while maintaining control over the characteristics of the layer.
For example, chemical vapor deposition (CVD) is a common deposition process employed for depositing layers on a substrate by introducing reactive precursors into a process chamber and allowing the precursors to react with the substrate. A variant of CVD that is being explored for its potential to demonstrate superior layer uniformity is atomic layer deposition (ALD). ALD comprises sequential steps of physisorption (physical absorption) or chemisorption (chemical absorption) that deposit monolayers of reactive precursor molecules on a substrate. A pulse of a first reactive precursor is introduced into a process chamber to deposit a first monolayer of molecules on the substrate. A pulse of a second reactive precursor follows to form an additional monolayer of molecules adjacent to the first monolayer. The additional monolayer typically reacts with the first monolayer to form the desired film. In this manner, a layer is formed on the substrate by alternating pulses of the appropriate reactive precursors into the process chamber. The cycle is repeated to form the layer to a desired thickness.
Both CVD and ALD require precise control of reactive precursors introduced into the process chamber in order to produce a desired layer of uniform thickness. For some applications of CVD and ALD, one or more of the precursors come in the form of a solid. Typically, the precursor changes state from a solid to a gas (vaporizes) at a certain pressure and temperature via a sublimation process carried out within a storage vessel. The solid precursor is delivered to the process chamber via a process gas produced by flowing a carrier gas through the vessel. (i.e., the process gas comprises the vaporized solid precursor mixed with the carrier gas). The rate of sublimation depends on a temperature of the solid precursor, a surface area of the solid precursor, and how the carrier gas flows through the vessel, each of which may be very difficult to control. Accordingly, it is often difficult to deliver a predictable amount of the solid precursor to the process chamber.
The difficulty in delivering a predictable amount of the solid precursor to the process chamber may lead to a number of problems. One problem is that irregularities in the amount of solid precursor delivered to the process chamber may result in nonuniformities in film thickness that adversely affect wafer quality and acceptability. A more fundamental problem, however, is that unless the amount of solid precursor that has been delivered to the process chamber is known, it is difficult to determine how much solid precursor remains in the vessel. It is therefore difficult to predict when the remaining solid precursor will be depleted. Typically, there is no simple method for measuring the amount of solid precursor remaining in the vessel. There is typically no visual indication of the level of the solid precursor through the vessel. Further, because the vessel is connected directly to a gas line, there is no simple method to weigh the vessel.
Current methods of predicting the amount of solid precursor remaining in a vessel range from a xe2x80x9cbest estimatexe2x80x9d approach based on experience to complex software algorithms that take into account various process variables, such as vessel temperature, vessel pressure and carrier gas volume flow rate. However, neither approach provides an adequately accurate prediction of depletion time and may result in replacing the solid precursor vessel prematurely or after complete depletion. Replacing the solid precursor vessel prematurely increases operating cost in the form of material waste and unnecessary down time. Replacing the solid precursor after complete depletion may result in wasted process cycles lacking solid precursor and scrapped wafers.
Therefore, a need exists for an improved method and apparatus for monitoring the delivery of solid precursor to a processing chamber.
Embodiments of the present invention provide an apparatus, method, and system for monitoring delivery of a precursor from a vessel to a process chamber via a process gas. The precursor is initially stored as a solid in the vessel prior to changing state to a gas through a sublimation process in the vessel.
The apparatus generally comprises a gas analyzer and a flow meter in communication with a controller. The gas analyzer generates a first signal indicative of a density of the precursor in the process gas. The flow meter generates a second signal indicative of a volume flow rate of the process gas or a carrier gas flowing into the vessel to produce the process gas. The controller calculates a mass flow rate for the precursor based on the first and second signals. Using the calculated mass flow rate, the controller may also calculate a total amount of precursor delivered to the process chamber and an amount of precursor remaining based on an initial amount. The gas analyzer may be any suitable gas analyzer and, for some embodiments, may comprise a Fourier transform infrared (FTIR) spectrometer.
The method generally comprises measuring a density of the precursor in the process gas, measuring a volume flow rate of the gas, and calculating a mass flow rate for the precursor based on the measured density of the precursor in the process gas and the measured volume flow rate of the gas. The method may further comprise calculating a total amount of precursor delivered to the process chamber, calculating an amount of precursor remaining based on an initial amount, and predicting an amount of time before the remaining precursor is depleted.
The system generally comprises a process chamber, a vessel containing a solid precursor to be delivered to the process chamber via a process gas produced by flowing a carrier gas into the vessel, and a solid precursor delivery monitor (SPDM) disposed between the process chamber and the vessel. The SPDM comprises a gas analyzer to generate a first signal indicative of a density of the precursor in the process gas and an integral controller to receive the first signal and a second signal indicative of a volume flow rate of the process gas or the carrier gas. The integral controller calculates the mass flow rate for the precursor based on the first and second signals and communicates the calculated mass flow rate to a system controller. Alternatively, the first and second signals may be routed directly to the system controller.